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E. Kalemci (Space Sciences Lab., UC Berkeley), J. A. Tomsick, R. E. Rothschild (CASS, UC San Diego), K. Pottschmidt (Integral Science Data Center), P. Kaaret (Harvard-Smithsonian, CfA)
I present results from studying the spectral and temporal evolution of all Galactic black hole transients (GBH) that have been observed with NASA's Rossi X-ray Timing Explorer during outburst decay. This study encompasses 11 sources and 14 outbursts. This is the first comprehensive and comparative study of GBHs during the transition to, and in the low/hard state (LS) with this many sources and outbursts. In this work I explored the physical conditions that lead to state transition and appearance of variability, characterized the quasi-periodic oscillations (QPO) and continuum of power spectral density (PSD) in different energy bands, and studied the correlations between spectral and temporal fit parameters. I also analysed the evolution of frequency-dependent and frequency-averaged time lags and coherence function during and after the transition.
Some notable results are summarized below. I show that broad band variability is observed only when the ratio of the power-law flux to the total flux in 3-25 keV band is greater than 0.5. This ratio, however, is not the only criteria to determine the appearance of broad band noise in the PSD. The energy dependent PSD analysis showed that the rms amplitude increases with energy when there is significant soft flux from disk and/or the absorption column density to the source is high. The spectral index shows a very tight correlation with both the QPO frequency and the total rms amplitude. The correlations between the characteristic frequencies of the PSD components are consistent with the previously reported ones. During the transitions, the average lags are higher and average coherence is lower. I interpret these results in the light of current models that explain the accretion structure and variability of these systems.
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Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.